使用数据库创建功能单元测试的标准/最佳实践是什么？

I get the idea behind unit testing however am trying to think of a clean simple way do it when it requires database functionality. For instance I have a function that return result based off a database select query. Would the database alway have to remain the same for me to properly see that only the correct results are being returned. What is the best way to perform unit testing (in PHP) when it requires database inactivity (whether it be read, write, update, or delete)?I get the idea behind unit testing however am t